The present invention relates to a process and an apparatus for the production of hollow bodies from thermoplastic material, the wall of which comprises a laminate formed by at least first and second layers, by means of extrusion blow molding.
In a process for the production of a hollow body from thermoplastic material with a wall in the form of a laminate structure comprising at least first and second layers, preforms are first produced in a batch-wise manner, the wall of each preform having a suitable number of layers, by using an extrusion unit comprising at least first and second continuously operating extruders with a common extrusion head. The extrusion unit is provided with at least one storage chamber for storage of materials which are plasticised in the extruders, together with at least one means which is movable with a reciprocating stroke-type movement between first and second limit positions by suitable drive means, for ejecting the materials from the storage chamber to empty same, thereby to form preforms. Flows of material from the at least one storage chamber, the number thereof corresponding to the number of layers for forming the wall of the hollow body, are brought together in the region of the extrusion head in such a way that mutually adjoining flows of material are joined together in at least one laminate-formation area to provide a laminate for constituting the wall of the preform. The laminate is advanced through a communicating duct within the extrusion head to an outlet opening disposed at a spacing from the storage chamber. A given portion by volume of the laminate is the ejected through the outlet opening to form the preform which has two end portions and a central portion. At least a part of the central portion is expanded within the mold cavity of a split blow molding mold under an increased internal pressure therein, while the end portions of the overall portion of material forming the respective preform are squeezed off the preform as constituting excess material, by means of a squeezing-off operation in which parts of the blowing mold participate, the excess material remaining outside the mold cavity.
It will be noted at this point that the means for emptying the storage chamber is generally in the form of an annular piston and for that reason, for the sake of simplicity, reference will be mainly made hereinafter to an annular piston, without however that term being intended to constitute a limitation in regard to the design configuration of the chamber-emptying means.
In an operating procedure along the lines of the process set forth above, as disclosed in German laid-open application (DE-OS) No. 36 35 334, the individual flows of material are firstly put into an annular configuration in cross-section, within the annular piston of the extrusion head, and the flows of material are then brought together within the annular piston to form the laminate. That laminate is passed through an annular duct into the storage chamber so that the plastic materials for making the preform are already stored in laminate form. Thus, in the subsequent emptying stroke movement for emptying the material from the storage chamber, which is performed by the annular piston, the laminate which has already been formed in the storage chamber is displaced towards the outlet opening thereof.
In another operating procedure as set forth in German laid-open application (DE-OS) No. 36 20 144, use is made of an extrusion head in which the flows of material from the extruders are passed through annular ducts disposed in a stationary component of the extrusion head, to a laminate-formation region from which the laminate produced from the flows of material is passed through an annular duct into the storage chamber which can then be emptied by the annular piston unit.
The fact that the preform is produced in a batch-wise manner in dependence on the stroke movements of the annular piston for emptying the material from the storage chamber in the above-discussed constructions, means that a distinction can be made between two operating phases, during a working cycle, within the extrusion head. After conclusion of the production of a preform by ejection of a suitable amount of laminate structure through the outlet opening of the extrusion head, the storage chamber is emptied. In that case the annular piston assumes its limit position at the end of the chamber-emptying operation. In the subsequent operating phase, namely the filling phase, the storage chamber is filled with laminate, the layers of which are conveyed into the extrusion head by the associated extruders which thus perform a material plasticising operation. In that case the annular piston used for emptying the storage chamber is then moved back into its second limit position corresponding to the end of the chamber-filling operation. During that operating phase, no preform is ejected from the extrusion head so that the material in laminate form which is to be found in the duct system between the storage chamber and the outlet opening is also not moved. After the operation of filling the storage chamber with material has been concluded, the procedure then consists of the other operating phase, namely the ejection or emptying phase, in which the previously formed laminate, for producing a preform, is ejected from the storage chamber under the action of the annular piston and displaced towards the outlet opening. When that happens, the laminate flows through a communicating duct towards the outlet opening. During that second operating phase, the ejection operation which involves the formation of the preform is predominantly effected by virtue of displacement of the material out of the storage chamber by means of the annular piston. However also added thereto is a minor portion of material which, when the extruders or plasticising devices are operating continuously, is conveyed by same into the extrusion head during the ejection stroke movement of the piston, with the consequence that as a result a certain proportion of material is additionally urged towards the outlet opening, in addition to the material which is displaced by the annular piston. Uninterrupted operation of the extruders for plasticising the materials for the layers of the laminate structure is desirable so that the operating conditions thereof can be stabilised.
The batch-wise and therefore discontinuous production of the preforms results in different operating conditions, corresponding to the different operating phases, in particular in regard to the pressure conditions obtained in the extrusion head. During the phase in which the storage chamber is being emptied, the laminate must be moved through the generally narrow outlet opening and therefore a high resistance to flow thereof has to be overcome. Accordingly in that phase the annular piston subjects the plastic material to an elevated pressure in order to be able to eject the laminate within a predetermined period of time.
During the other operating phase when the storage chamber is being filled, the pressure obtained in the plastic material is usually low. As the outlet opening is generally not a closable opening, it is only possible in that way to provide that no laminate flows out of the outlet opening as the pressure is too low to overcome the resistance to flow thereof.
The above-indicated different pressure conditions which accordingly vary from one phase to another mean in relation to the known procedures that, during the transitions from filling to emptying and from emptying to filling, different pressures obtained in the laminate-formation region in the individual flows of material which make up the laminate structure, and those different pressures can result in irregularities in formation of the laminate structure. Those irregularities therefore occur in particular in the portion of the laminate which is formed in the laminate-formation region during the period of a transition between two operating phases, which can be extended to the final phase of the emptying procedure and the starting phase of the filling operation, or the final phase of the filling operation and the starting phase of the emptying operation. That is to be attributed for example to the fact that thermoplastic materials in a plastic condition do not behave like a liquid but within certain limits have a certain degree of resilient compressibility which, when the pressure is relieved, results in a return or restoration movement, that is to say an increase in volume while, when the pressure is increased, the resilient compressibility results in a compression effect and therefore a reduction in the volume of the plastic material, so that it is necessary to bear in mind that, during the transitions between the operating phases, in the region in which the laminate is being formed, the individual flows of material for making up the laminate structure experience deformation effects due to differences in compression and decompression in respect of the individual flows of material. Such deformation of the flows of material will result in the laminate being of an irregular structure, in particular in regard to spatial and quantitative distribution of the individual layers in the laminate structure. At any event a relief of pressure can result in decompression of the material and an increase in pressure can result in compression of the material; due to the different volumes involved in the individual layers and the generally different volumes of the individual duct systems through which the flows of material, while still separated, flow to the laminate-formation region where they are brought together to form the laminate structure, and in consideration of differences in the properties of the materials forming the individual flows of material, the compression and decompression effects may be different in the respective individual flows of material, with the result that irregularities in the distribution of the individual layers in the laminate structure may occur in the region of the laminate-formation area, in the individual flows of material going to that area. Those irregularities do not occur at least to that extent when the preforms are being extruded continuously as that procedure does not involve variations in the pressure conditions, as are caused by batch-wise formation of the preforms.
The above-described irregularities can result in serious reductions in the level of quality of the hollow bodies to be made from the preforms, by virtue of the fact that the walls thereof are of an irregular nature. In that connection the nature and extent of the irregularity can be neither controlled nor foreseen. In that respect the way specifically in which the flows of material are guided in the extrusion head prior to the laminate formation operation is immaterial as the variations in the pressure conditions, which give rise to those irregularities, do at any event occur due to the change between the filling and the emptying phases, which is a typical situation in regard to batch-wise production of the preforms.
In many situations, in regard to the production of hollow bodies by means of extrusion blow molding, it is expedient and possibly even necessary for the wall thickness or gauge of the preform to be influenced during the material ejection operation, for example in such a way that the regions of the preform which, upon subsequent expansion thereof under the effect of an increased internal pressure, are subjected to a substantial stretching effect, are of greater wall thickness or gauge than those parts of the preform which are only slightly expanded by virtue of the increased internal pressure. In that way it is possible to produce hollow bodies which comply with individual requirements, in particular with a substantially uniform wall thickness. That is achieved by the cross-section of the generally annular outlet opening being enlarged or reduced in an appropriate fashion, for example by means of a movable internal component operatively associated therewith. However those increases and reductions in the dimension of the outlet opening also result in a change in the flow resistance to which the material is subjected and accordingly result in fluctuations in pressure, which can possibly also have a disturbing influence on the formation of the laminate, with the result that the quality of the hollow body produced therefrom is also adversely affected.
Furthermore changes in certain properties of the plasticised material which is frequently formed from a blend of different plastic materials and/or fillers, as well as variations in temperature, may result for example in fluctuations in viscosity which affect the flow properties of the plastic material and thus cause fluctuations in pressure, in particular during the storage chamber-emptying phase. Those fluctuations are generally unforeseeable and unavoidable.